The current invention is related to a method of and an apparatus for substantially reducing an optical surge which is caused by an optical amplifier during a switch between optical lines.
In the recent years, as the global data communication such as the Internet becomes readily available, improvements in the capacity and the speed of the data communication between terminal devices are highly desirable. In order to accommodate such demands, optical communication systems including optical fibers and operational control devices have been introduced. Since the optical lines carry an increased amount of data per line, the optical communication systems are generally more susceptible to a large amount of data loss due to a problem in the communication line.
In order to improve the reliability of the optical communication system, one implementation includes a duplicate pair of optical lines and an optical switch device for switching from one optical line to another in case of a line problem. Optical switches are generally grouped into a mechanical type, a frequency type and a wave guidance type. The mechanical type switch mechanically moves an optical fiber or a lens to switch from one line to another. The frequency type switch includes an element such as an optical filter to cause the switch. Lastly, the wave guidance type switch includes an element having material such as polymer, semiconductor or quartz. This implementation not only increases reliability, but also simplifies over the prior art communication systems where an additional device is needed for converting an optical signal to an electrical signal in case of a line problem. The simplification also reduces the physical size as well as the cost for implementing an optical communication system.
Another optical device to increase the reliability of the optical data communication system is an optical amplifier. Since an optical signal is attenuated by a number of factors such as the distance of an optical line, an optical switch, a splitter used in a wavelength division multiplier, an optical filter and so on, the attenuated optical signal must be amplified by a series of optical amplifiers along the optical line. If an optical signal is failed in one optical line, the optical switch changes lines to maintain the signal level by properly amplifying the optical signal. Examples of a combination of the above described optical switch and optical amplifier are disclosed in publications such as xe2x80x9cPostdeadline Papers, PD25xe2x80x9d Optical Fiber Communication Conference, 1998. Another paper, B-1083 in Academy of Electronics Information Telecommunication (1996) discloses a multiple-line optical cross connect system including an optical monitor, a 4xc3x974 optical switch and a control unit.
The combined use of an optical switch and an optical amplifier may cause an optical surge during a switch transition. There is a finite delay in time when the optical switch closes a contact from one optical line to another. During the delay, the newly switched line experiences the lack of an optical signal and then a sudden increase in the optical signal which is fed into an optical amplifier. This set of events causes an optical surge in the output of the optical amplifier. The optical surge may be worsened by further amplification by a series of optical amplifiers. The optical surge generally damages certain components such as optical receivers and connectors in the optical data communication system and interrupts the optical signal transmission. Furthermore, the optical surge may harm the human operator of the optical data communication system. For these and other reasons, the optical surge must be substantially reduced to eliminate the undesirable effects.
In order to suppress an optical surge, prior attempts have included different methods. Prior art references, B-941 Spring Session, Academy of Electronics Information and Telecommunication, 1993 and B-10-177, Conference of Telecommunication Society, Academy of Electronics Information and Telecommunication, 1997 disclose delaying an optical input to an optical line. However, these prior art disclosures fail to consider an optical amplifier in an optical system and lack any consideration for applying the technique to an optical system having an optical amplifier. Another prior art reference, B-10-176, Conference of Telecommunication Society, Academy of Electronics Information and
Telecommunication, 1997 discloses a technique of shutting off of an excitation light source for an optical amplifier upon terminating an input to the optical amplifier. This prior art technique lacks disclosures on the timing of the termination of the excitation light source with respect to the onset of switching optical lines. Lastly, the same B-10-176 prior art reference also discloses a technique for inputting a control light into an optical amplifier. This prior art technique not only lacks disclosures on the timing of inputting the control light with respect to the onset of switching optical lines, but also requires additional expensive pieces of equipment for emitting the control light.
A cost-effective method and device for substantially reducing an optical surge is highly desired.
In order to solve the above and other problems, according to a first aspect of the current invention, a method of reducing an optical surge caused by optical amplifiers while switching from a first optical line to a second optical line, including the acts of: detecting a predetermined line switching signal indicative of switching the first optical line to the second optical line; altering an amplification gain of at least one of the optical amplifiers towards a predetermined safe gain range upon detecting the predetermined line switching signal; detecting that the amplification gain is within the predetermined safe amplification gain range; and switching from the first optical line to the second optical line upon detecting the predetermined safe gain range, wherein undesirable effects of the optical surge is substantially reduced to an acceptable level.
According to a second aspect of the current invention, a method of reducing an optical surge caused by optical amplifiers while switching from a first optical line to a second optical line, including the acts of: detecting a predetermined line switching signal indicative of switching the first optical line to the second optical line; initiating a change in amplification gain at a predetermined rate in at least one of the optical amplifiers towards a predetermined safe value upon detecting the predetermined line switching signal; timing an elapse of a predetermined amount of time after the initiating act so that the amplification gain at least reaches the predetermined safe value; and switching from the first optical line to the second optical line after the elapse of the predetermined amount of time, wherein an amount of the optical surge is substantially reduced for undesirable effects.
According to a third aspect of the current invention, an optical surge reduction apparatus including: an optical switch for switching between a first optical line and a second optical line in response to a switch drive signal so as to maintain an operational optical line, the first optical line and the second optical line concurrently transmitting a duplicate set of an optical signal; an optical amplifier connected to the optical switch for amplifying the optical signal in the operational optical line at a variable amplification gain; and a control unit connected to the optical switch and the optical amplifier for detecting an optical line failure condition and for changing the variable amplification gain to a predetermined safe amplification gain value in response to the optical line failure condition prior to generating the switch drive signal.
According to a fourth aspect of the current invention, an optical surge reduction unit, including: an optical switch for switching in response to a switch drive signal between a first optical line and a second optical line so as to route an optical signal via either the first optical line or the second optical line; a pair of optical amplifiers each connected to the first optical line and the second optical line for amplifying the optical signal at a variable amplification gain; and a control unit connected to the optical amplifiers and the optical switch for controlling the variable gain to a predetermined safe amplification gain value in response to a routing change signal prior to generating the switch drive signal.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.